BUS TRANSCEIVER WITH BINARY DATA TRANSMISSION MODE AND TERNARY CONTROL TRANSMISSION MODE

Abstract: The present invention provides a bus transceiver incorporating a high speed, binary transfer mode for the half-duplex transfer of data signals with a ternary control transfer mode having a full duplex dominant logic transmission scheme for the full duplex transfert of control signals. In one embodiment of the present invention, the above-noted transfer modes are implemented in a bus architecture which includes at least a first communications node coupled to a second communications node via a twisted pair, serial bus. Each node comprises first transceiver and second transceivers having a differential driver for driving on the bus signal states comprising first and second signal states having equal current amplitudes opposite in sign and a third signal state having approximately a zero current amplitude, a high speed binary receiver for receiving high speed data signals during data transfer phases and a ternary receiver for receiving control signals during control transfer phases. The ternary receiver comprises two binary receivers for detecting resultant current amplitudes created on the bus during simultaneous driving of control signals by the nodes during the control transfer phases and logic means for combining the resultant current amplitudes on the bus with the signal states driven by the local transceiver to output reconstructed control signals representing the control signals driven on the bus by the corresponding transceiver. Furthermore, both transceivers further include a preemptive signaling receiver for the detection of preemptive control messages which act to terminate the data transfer phases upon receipt of the message so that higher priority control transfers may take place.

Description: BUS TRANSCEIVER WITH BINARY DATA TRANSMISSION MODE AND TERNARY CONTROL TRANSMISSION MODE BACKGROUND OF THE INVENTION ( l ) Field of the Invention The present invention relates to a bus architecture communications scheme for enabling baseband data communications between a plurality of interconnected devices or nodes in a computer system, and more particularly, to a bus transceiver incorporating a high speed, binary transfer mode with a low speed, ternary control transfer mode having a full duplex dominant logic scheme for bi¬ directional, simultaneous signal transfer. Hereinafter, all computer devices will be referred to as "nodes" for simplicity irregardless of whether or not the specific device has a higher intelligence otherwise known as a "local host". In addition, the term local host will be used irregardless of whether or not it comprises hardware or hardware and software. Nodes within a computer system, such as a disk drive, a CRT, a printer and the like, need the ability to convey signals between themselves. In the past, this has been accomplished by means of a standard I/O bus which comprises a plurality of transmission lines or channels and acts as a shared communications path for interconnecting several nodes in the system. In such a system, it is desirable to provide a bus architecture in which signals can be simultaneously transmitted between nodes connected to the bus in order to increase the volume of information that can be transmitted in a given time period, thereby increasing the over-all speed of the computer. This type of transmission is known as full-duplex transmission. In conventional bus architectures, however, a signal transmitted on the bus by a particular node is available for reception by all other nodes attached to the bus so that if two or more signals were simultaneously transmitted on the bus, they would superimpose to create a garbled signal un-intelligible to any node. In conventional baseband data communication systems, this problem has been partially overcome by the following two techniques. The first technique is known as time duplexing wherein a plurality of nodes connected to a shared I/O bus are individually given a specific allotment of time to transmit their signals on the bus. This type of transmission resembles a round robin procedure in which each node is sequentially given access to the bus. A major drawback with the use of time duplexing on a shared bus is that it requires a general synchronization system to sequentially enable the driver of each node for a certain time, and even then, the over-all transmission is slow since the signals are still individually transmitted. Furthermore, this type of transmission is not bi-directional (i.e., full duplex) so that a node which requests information from another node must wait until the second node is given its turn to transmit the desired information. The second technique consists of providing a bus which has at least two transmission lines for each interconnection between nodes coupled to the bus so as to enable full duplex transmission between any two of the nodes. For example, if there are four nodes coupled to each other so as to form six interconnections, twelve transmission lines would be required. Although this technique solves the problems presented by the previous technique, it creates other problems relating to the physical size and complexity of the bus and the large amount of power required to drive the bus. Furthermore, in prior art bus architectures, data transfer busses are commonly used for the transmission of baseband data signals between nodes directly coupled to each other via point-to-point interconnections or "links" forming the bus. In this type of bus, data signal transfers between nodes occur only occasionally when one node needs to request data from or transmit data to another node. Because the bus may idle for indefinitely long periods of time between successive data transfers, it is customary to put the bus in an idle state by disabling the drivers of adjacent nodes so as to reduce the power consumption that would otherwise occur. One method of maintaining the bus in the idle state is to use bias circuits to actively bias the bus in one of two signal states (i.e., a binary 1 or 0 signal state), however, this approach still consumes a significant amount of power due to the current required to maintain these states. Another known method for use with a differential data transfer bus is to passively bias the bus through the use of low impedance, passive termination devices. However, the use of this method to maintain the bus idle state undermines the overall common mode rejection of a double terminated, differential bus. Hence, in order to maintain the bus idle state, prior art bus architectures have traditionally required either the utilization of large amounts of power or a sacrifice in the common mode rejection range of the bus. More recently, however, another means for reducing the power consumption caused by the bus idle state was realized through the use of a third, low power signal state as the bus idle state. This third state, known as a Z signal state, has a signal amplitude half way between the two binary signal states such that the corresponding current amplitude (when driven on the bus) is nearly 0 mA. According to the present invention, it was realized that the use of an intermediate signal state having a current amplitude of approximately 0 mA for the bus idle state would not only cause a reduction in the power consumption by the bus during an idle state, but would also provide an additional signaling state for use in implementing a full-duplex transmission scheme on the bus. Although this third state has been used in burst mode, differential busses for reducing the power consumption during the bus idle state, it has not been utilized for the transfer of data on a bus. In an attempt by Applicants to implement the Z signal state in a burst mode, differential bus, it was found that this state constituted a special problem due to the design of conventional binary receivers utilized in such busses for the detection of the binary signal states. Specifically, a naive implementation of the intermediate Z signal state on the bus using a conventional binary receiver causes unpredictable behavior and produces erroneous signals during the bus idle state. This is due to current fluctuations on the bus about the intermediate current amplitude of the Z signal state which causes the receiver to randomly switch states between the binary signal states. According to another implementation, Applicants utilized a hysteresis receiver having a hysteresis region around the current amplitude of the Z signal state to prevent the random oscillation. However, the use of a hysteresis receiver, while absolutely mandatory during the control signal transfer phase, represented a serious problem during the data signal transfer phase where the hysteresis receiver generated adverse duty cycle distortions during normal data transmissions. Accordingly, it is an object of the present invention to provide a method and apparatus for utilizing the Z state in conjunction with the binary states of a bus as a signal state for maintaining the bus in an idle state and as a bus state for implementing a full-duplex, dominant logic transmission scheme. Another object of the present invention is to provide a method and apparatus for a bus transceiver incorporating a high speed, binary data transfer mode with a ternary control transfer mode having a full duplex dominant logic transmission scheme for automatic determination of the preferred data signal transfer direction and for the exchange of other control and identification information. A further object of the present invention is to provide a method and apparatus for preemptive signaling in a half-duplex binary transmission scheme wherein a driver of one transceiver is interrupted by the driver of another transceiver for the transmission of a higher priority message. Still a further object of the present invention is to provide a method and apparatus which implements the above objects in a twisted pair, differential serial bus interconnecting transceivers of two nodes such as the one proposed in the IEEE PI 394 standard. BRIEF SUMMARY OF THE INVENΗQN The present invention provides a bus transceiver incorporating a high speed, binary transfer mode for the half-duplex transfer of data signals with a ternary control transfer mode having a full duplex dominant logic transmission scheme for the full duplex transfer of control signals. In one embodiment of the present invention, the above-noted transfer modes are implemented in a bus architecture which includes at least a first communications node coupled to a second communications node via a twisted pair, serial bus. Each node comprises a first transceiver and a second transceiver such that one pair of twisted signal wires interconnects the first transceiver of the first node with the second transceiver of the second node, and the second pair of twisted signal wires interconnects the first transceiver of the second node with the second transceiver of the first node. Both the first and second transceivers comprise a differential driver for driving differential signal states on the bus, a high speed binary receiver for receiving high speed data signals and a ternary receiver for receiving control signals. Furthermore, both of the second transceivers also include a preemptive signaling receiver for the detection of preemptive control messages which act to terminate the current data transfer phase upon receipt of the message. With respect to the binary data transfer mode, the binary receivers comprise high speed receivers having a single voltage comparator for the reception of data signals. The voltage comparators translate the bus states into corresponding signal states during the data transfer mode. Since the high speed, binary data transfer mode is only half-duplex, an arbitration scheme having full duplex transmission capabilities is required to determine the preferred direction of the binary data transfer. This is performed by implementing a dominant logic scheme in the ternary receiver of each transceiver which provides the simultaneous transmission capability. The dominant logic scheme utilizes at least three signal states present on the bus, wherein in a preferred implementation, one of the signal states is the intermediate Z signal state having a current amplitude of approximately 0 ma. The Z signal state is utilized not only to reduce the power consumption of the bus during a bus idle state (by virtue of its almost null current amplitude), but also to increase the amount of information that can be transferred on the bus. This latter function is made possible through the use of the ternary receiver in each transceiver which operates pursuant to the dominant logic scheme. In implementing the dominant logic scheme, the three signal states are combined on the bus through the simultaneous driving of selected signal states by each of the transceiver pairs so as to form one of three bus states representing the resultant current amplitude on the bus. The bus state formed on the bus is then detected and utilized by the ternary receivers to create one of three corresponding encoded logic states which is then partially decoded to obtain an output representing to a certain extent the signal states being driven by the corresponding transceiver. These partially decoded logic states are subsequently utilized by the receiving communications node for a variety of control functions in the arbitration phase of the bus. In one embodiment of the present invention, a preemptive - signaling receiver is provided comprising either a voltage comparator or an operational amplifier in the clock transmitting transceiver of each node. When the first node must transmit priority control information to the second node during a data transfer from the second node to the first node, the first node enables the driver of the clock receiving transceiver so that a preemptive message is driven on the signal lines transmitting the reference clock signal. In this manner, the dc level of the reference clock signal is shifted up to a higher level to trigger the threshold of the comparator of the preemptive signaling receiver. The preemptive receiver then outputs a corresponding preemptive signal to the driving circuitry of the second node in order to stop the transfer of data to the first node. Furthermore, in order to permit the voltage comparators forming the various receivers of the present invention to receive the transmitted signals at the amplitude required to detect the proper bus voltage values, the present invention further provides a means for common mode shifting of the signals at the front end of the receivers. This is performed by providing each receiver of the transceivers with a level shifting circuit having PNP bipolar junction transistors implemented as a common mode voltage translator at the front end of each voltage comparator. According to a further embodiment of the present invention, a method and apparatus is provided for the full duplex transmission of arbitration signals in a particular multi-speed bus architecture having a plurality of nodes arbitrarily coupled together via a plurality of links to form an interconnected, multi-speed bus. Each node coupled to the interconnected, multi-speed serial bus comprises a physical channel interface chip (or "PHY") directly connected to the serial bus for the reception and transmission of control and data signals. Each node may also comprise a link layer chip (or "LLC") which transmits to and receives from the PHY both data and control signals via a fixed speed, variable size bus Y forming a scalable interface. With such an arrangement, the interconnected pairs of bus transceivers of the present invention are respectively utilized in adjacent nodes of the system. In this manner, the transmission of data signals between PHY's of adjacent nodes is performed using the binary transfer mode and the transmission of control signals is performed using the ternary transfer mode as described above. BRIEF DESCRIPTION OF THE DRAWINGS The objects, features and advantages of the present invention will become apparent from the following detailed description in which: Figure 1 is a block diagram showing two pairs of bus transceivers utilized for a differential serial bus implementation of the present invention wherein one pair of data/arbitration signaling transceivers comprise a binary receiver and a ternary receiver and another pair of clock signaling transceivers comprise a binary receiver, a ternary receiver and a preemptive signaling receiver. Figure 2 is a diagram of the PNP level-shifting circuit used at the front end of each of the receivers of the bus transceivers of the present invention. Figure 3 is a block diagram showing a plurality of nodes interconnected in an arbitrary bus topology pursuant to the P1394 bus architecture standard, including the presentation of a scalable interface (denoted as the bus Y) between the PHY and LLC chips of a particular node, wherein the bus transceivers of the present invention are incorporated in the PHY's of adjacent nodes. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for a bus transceiver incorporating a high speed, binary transfer mode with a ternary control transfer mode having a full duplex dominant logic transmission scheme. In the following description, numerous details are set forth such as device speeds, device types, operating modes, etc., in order to provide a thorough understanding of the present invention. It will be obvious to one skilled in the art, however, that these details are not required to practice the present invention. In other instances, well-known circuits, devices, methods and the like are not set forth in detail in order to avoid unnecessarily obscuring the present invention. In a first embodiment of the present invention as shown in Fig. 1, two pairs of bus transceivers 1,22 and 2,21 are interconnected via a twisted pair, serial bus 45 for the half duplex transmission of data signals and the full duplex transmission of control signals between nodes 51,52. The twisted pair, serial bus 45 comprises two pairs of signal wires 40,41 and 42,43 with each pair forming a transmission channel 20 for the transmission of differential signals. During a data transfer phase on the bus 45 where data is transmitted from the first node 51 to the second node 52, the first pair of signal wires 40,41 is used to transfer differential data signals while the second pair of signal wires 42,43 is used to transfer differential clock signals between the two nodes 51,52. Similarly, during a data transfer phase on the bus 45 where data is transmitted from the second node 52 to the first node 51, the first pair of signal wires 40,41 is used to transfer differential clock signals while the second pair of signal wires 42,43 is used to transfer differential data signals between the two nodes 51,52. In an arbitration phase, however, the bus 45 is used in a full duplex mode (as a two bit parallel channel, for example) where both the first and second nodes 51,52 transmit arbitration information simultaneously on both pairs of signal wires 40,41 and 42,43. Although the first embodiment is directed toward an application of the bus transceiver with a serial bus 45 having twisted pair signal wires 40,41 and 42,43, it would be obvious to one skilled in the art to utilize the bus transceivers and the dominant logic scheme of the present invention in any type of bus which performs baseband data transfers despite the number of transmission channels in the bus, the type of communication channel (i.e., single ended or differential) or the specific number of signaling levels used. Similarly, although the first embodiment depicts the bus 45 as being simply coupled between two nodes 51,52, in another embodiment to be described below (see Fig. 3), a plurality of nodes 3 in a computer system are arbitrarily coupled together via a plurality of the twisted pair busses 45 comprising links 40 to form, in general, an interconnected, multi- speed bus 80. With reference to the first and second transceivers 1,2 shown in Fig. 1, each comprises a differential driver 4 for driving differential signal states on the bus 45, a high speed binary receiver 5 for receiving high speed data signals and a ternary receiver 6 for receiving control signals. With reference to the transceivers 21 and 22, each similarly comprises a differential driver 4, a high speed binary receiver 5 and a ternary receiver 6. However, transceivers 21 and 22 additionally comprise a preemptive signaling receiver 7 to detect preemptive control messages superimposed on the clock signal wires by the respective receiving node during a data transfer phase for terminating the transfer of data upon receipt of such a message. Both the driver 4 of transceiver 1 and the driver 4 of transceiver 2 can drive data signals or control signals depending upon the state of the input multiplexors 2 which supply the signals as input to the drivers 4. Separate enable multiplexors 3 supply the correct enable control signals depending upon the transceiver state (data or control signal transfer). In a preferred implementation of the driving circuit, both the data and control lines are two bits wide. The first bit enters the driver input multiplexor 2 and controls the signal states of the driver 4. The second bit enters the driver enable multiplexor 3 and controls the on/off states of the driver 4. During the data signal transfer phase, only one driver 4 of both transceivers 1,2 is on; the enabled driver remains on while the disabled driver remains off until the termination of the data signal transfer phase. (An exception to this is the transmission of a preemptive control message (during the preemption of a data transfer phase) by the driver 4 of the transceiver receiving the clock signals during the particular data transfer phase as will be more fully explained below.) During the control signal transfer phase, each driver 4 is enabled or disabled as necessary. A disabled driver generates the Z signal state, while an enabled driver generates the binary signal states directed by the first input bit. To enable the full duplex transmission of control signals, including arbitration signals, between the first and second nodes 51,52, the present invention provides a bus 45 having at least three signal states A, wherein in a preferred implementation, the third signal state is the intermediate Z signal state having a current amplitude of approximately 0 mA. The Z signal state is utilized not only to reduce the power consumption of the bus 45 during a bus idle state (by virtue of its almost null current amplitude), but also to increase the amount of information that can be transferred on the bus 45. This latter function is made possible through the use of the ternary receiver 6 in each transceiver 1,2,21,22 which operates pursuant to a dominant logic scheme. In implementing the dominant logic scheme, the three signal states A (0,1,Z) are combined on the bus 45 through the simultaneous driving of selected signal states A by each of the transceiver pairs 1,22 and 2,21 so as to form one of three bus states B (0,1,Z) representing the resultant current amplitude on the bus 45. The bus state B formed on the bus 45 by a simultaneous driving situation is then detected and utilized by the temary receivers 6 to create one of three corresponding logic states D (0,1,Z) which is subsequently decoded by the ternary receiver 6 of one transceiver to determine the signal state A being driven by the other transceiver so as to reconstruct the control message transmitted by thereby. Although the dominant logic scheme is preferably implemented through the use of three signal states A, the present invention can clearly be extended to transmission channels which can accommodate more than three signal states A, and hence, more than three bus states B which can be used to form more than three logic states D. Similarly, the invention can also be implemented using a third signal state X different from the intermediate Z signal state. This can be done as long as at least three signals states A are utilized to form three bus states B such that: (al) The current amplitude resulting on the bus 45 from the simultaneous transmission of a 0 signal state by one driver 4 and a 1 signal state by the corresponding driver 4 is approximately the same as the simultaneous transmission on the bus 45 of the X signal state by both drivers 4 (i.e., an X bus state); (a2) The current amplitude resulting on the bus 45 from the simultaneous transmission of a 0 signal state by one driver 4 and the X signal state by the corresponding driver 4 is approximately the same as the transmission of the 0 signal state by any one of the two drivers 4 (i.e., a 0 bus state); (a3) The current amplitude resulting on the bus 45 from the simultaneous transmission of a 1 signal state by one driver 4 and the X signal stage by the corresponding driver 4 is approximately the same as the transmission of the 1 signal state by any one of the two drivers 4 (i.e., a 1 bus state); (a4) The current amplitude resulting on the bus 45 from the simultaneous transmission of a 0 signal state by both drivers 4 is approximately the same as the transmission of the 0 signal state by any one of the two drivers 4 (i.e., a 0 bus state); (a5) The current amplitude resulting on the bus 45 from the simultaneous transmission of a 1 signal state by both drivers 4 is approximately the same as the transmission on the bus of the 1 signal state by any one of the two drivers (i.e., a 1 bus state); and (a6) The current amplitude resulting on the bus 45 from the simultaneous transmission of the X signal state by both drivers 4 is approximately the same as the transmission of the X signal state by any one of the two drivers 4 (i.e., an X bus state). This combination of signal states A used to create the bus states B is summarized in Table 1 below: Iab l First signal state A Second signal state A Bus state B 1 1 1 1 0 Z 1 Z 1 0 1 z 0 0 0 0 z 0 Z 1 1 z 0 0 z z z In the preferred implementation, the transmission of the Z signal state on the bus 45 is performed by disabling the corresponding driver 4 of the data transmitting receiver, thereby reducing the overall power consumption while automatically fulfilling the above conditions a2, a3, a4, a5 and a6. Condition al can then be fulfilled by choosing the current amplitudes of the binary signal states 0 and 1 such that the resulting current amplitude (or bus state B) during a simultaneous driving of the 0 and 1 signal states is almost null so as to form the Z bus state. In order to detect the bus states B formed on the bus 45, the ternary receivers 6 each comprise two voltage comparators 11,12 having built in voltage thresholds or operational amplifiers. Furthermore, the ternary receivers 6 further comprise logic circuitry for translating the bus states B into corresponding logic states D and E which are subsequently decoded by the ternary receivers 6 for use in the transfer of control messages during the arbitration phase on the bus 45. In terms of a specific implementation of the three signal states A on a differential serial bus 45 in which each pair of signal wires 40,41 and 42,43 has a positive wire 40,42 and a negative wire 41,43, respectively, a binary 0 signal state is transmitted on the transmission channel 20 by driving -4 mA on the positive wire and +4 mA on the negative wire; a binary 1 signal state is transmitted on the transmission channel 20 by driving +4 mA on the positive wire and -4 mA on the negative wire; and the Z signal state is transmitted on the transmission channel 20 when approximately 0 mA exists on each of the signal wires. With respect to the bus states B produced on the transmission channel 20 by the simultaneous driving of signal states A, there are in essence five possible bus states: A 00 bus state corresponding to a current amplitude of approximately -8 mA on the positive wire and approximately +8 mA on the negative wire; a 0 bus state corresponding to approximately -4 mA on the positive wire and approximately +4 mA on the negative wire; a Z bus state corresponding to approximately 0 mA on both wires; a 1 bus state corresponding to approximately +4 mA on the positive wire and approximately -4 on the negative wire; and a ll bus state corresponding to approximately +8 mA on the positive wire and approximately -8 mA on the negative wire. Furthermore, in the preferred implementation, the transmission channels 20 have a characteristic impedance of 110Ω and are double terminated. Therefore a 0 bus state is represented by a differential voltage of approximately - 220 mV. Similarly, a 1 bus state is represented by a differential voltage of approximately +220 mV. Accordingly, a Z bus state is represented by a differential voltage of approximately 0 mV. Although it is possible to produce five bus states B on the transmission channel 20, the preferred embodiment of the present invention is designed to take advantage of only three out of the five possible bus states B (i.e., 0, 1 and Z) in order to minimize the complexity of the transceiver circuitry. However, it would be obvious to one skilled in the art that all five bus states B can be utilized to extract more information for use in a higher level bus protocol. The use of all possible bus states B would require the addition of a multi¬ level encoding mechanism (already in use) in addition to the implementation of two more voltage comparators 11,12 (or operational amplifiers) in the ternary receivers 6. As mentioned above, the ternary receiver preferably comprises two voltage comparators for detecting the bus states B formed on the bus 45. The threshold of the first comparator 11 therefore has a value between the detected voltage amplitudes of the Z bus state and the 1 bus state (i.e., -98 mV), while the threshold of the second comparator 12 has a value between the detected voltage amplitudes of the Z bus state and the 0 bus state (i.e., +98 mV). Furthermore, with respect to the utilization of the ternary receiver 6 to implement the three signal states A in the present invention, it was necessary to realize a fundamental distinction between the data transfer mode of operation and the arbitration mode of operation on the transmission channel 20. The binary data transfer mode is in general a half duplex transmission where its speed is limited only by the capability of the transceiver circuits and the bandwidth of the transmission channel 20. The transceiver of the data transmitting node outputs on the transmission channel 20 the desired message at the maximum possible speed independent of the signal propagation delay along the transmission channel 20. The arbitration transfer mode, however, is in general a full duplex transmission where the maximum possible speed is largely limited by the propagation delay along the transmission channel 20. This is because it is necessary to provide all nodes 3 coupled to the interconnected bus 80 with a chance to see and respond to the arbitration messages. Therefore, the full duplex transmission provided by the ternary receivers 6 is necessary only during the arbitration transfer mode. The fundamental limits that the bus' physical configuration imposes upon the arbitration process enables the use of lower bandwidth ternary receivers 6 without significantly impacting the overall performance of the system. The lower bandwidth temary receivers 6 will compensate for the reduction in signal amplitude, thus maintaining the same bit error rate levels. With respect to the binary data transfer mode, the binary receivers 5 comprise high speed receivers having a single voltage comparator 13 for reception of data signals transmitted via the transmission channel 20. The voltage comparators 13 translate the bus states 0 and 1 into the signal states 0 and 1 during the data transfer mode. The higher level protocol controlling the operation of the entire bus 80 shown in Fig. 3 guarantees that during the data transfer mode on the bus 45, the only bus states B used are 0 and 1. Hence, the binary receivers 5 of the present invention do not require hysteresis, thereby minimizing the duty-cycle distortion of the received data. During the bus arbitration phase, however, the receivers 5 are disabled since the control signals are received only by the temary receivers 6. Since the high speed, binary data transfer mode is only half- duplex, an arbitration scheme is required to determine the preferred direction of the binary data transfer. In the most general case, the negotiation to determine the preferred direction of the upcoming data transfer can involve all the nodes 3 connected to the bus 80. Because the exchange of control signals which implement the bus arbitration algorithm is made full duplex, the duration of the arbitration phase is reduced, thus maximizing the bus useful bandwidth. The dominant logic scheme implemented in the ternary receivers 6 enables the transceivers connected to the links 40 to simultaneously transmit and receive control information, thus reducing the minimum duration of the arbitration phase. In the implementation of the dominant logic scheme in the ternary transfer mode, the ternary receiver 6 of a "data receiving" transceiver 21 utilizes two voltage comparators 11,12 each able to output a binary value C responsive to the detected bus state B. With reference to Fig. 1 and Table 2 below, through the use of combinational logic, such as a first multiplexor 8, the two binary values C derived from the first and second comparators 11,12 are translated into a logic state D representing the bus state B on the transmission channel 20. This logic state D is then supplied as input to a second multiplexor 9 along with the signal state A being driven locally by the driver 4 of the data receiving transceiver 21 to output a partially decoded logic state E representing to a certain extent the signal state A transmitted by the driver 4 of the "data transmitting" transceiver 2. (It is noted that although each transceiver is driving a signal state A on the transmission channel 20 so as to produce a simultaneous driving situation, we refer to a specific transceiver as a "data receiving" transceiver (either 21 or 22) when we are analyzing its decoded output to determine what signal state A the corresponding "data transmitting" transceiver (either 2 or 1, respectively) is driving on the transmission channel 20.) As noted above, although the logic state E output by the data receiving transceiver 21 is only partially decoded, the logic state E can be fully decoded if all five bus values B were utilized along with the implementation of two more voltage comparators in the ternary receivers 6. In the preferred embodiment, however, it was decided to select a minimum complexity implementation for practical purposes and to work around the four situations in which the logic states E output by the ternary receivers 6 could not be fully decoded. This is done by adopting an arbitration scheme for the interconnected bus 80 of the present invention in which a differentiation between the possible signal states A represented by a particular logic state E is unnecessary. That is, in terms of the arbitration message exchange on the interconnected bus 80, the logic states E are utilized in the arbitration scheme in such a manner so as to be indistinguishable to the data receiving transceiver 21. With respect to the specific implementation of the temary receiver in each of the transceivers, the first comparator 11 of the ternary receivers 6 has a threshold of M (i.e., approximately +98 mV) such that it is able to determine from an input voltage I of a detected bus state B either that I ≥ M or that I < M, where the comparator 11 outputs as a binary value a 1 or a 0, respectively (see Table 2 below). The second comparator 12 having a lower threshold of N (i.e., approximately -98 mV) is able to determine either that I ≥ N or that I < N, where the second comparator 12 outputs as a binary value a 1 or a 0, respectively. In this manner, the bus states B existing on the bus 45 during the simultaneous driving situations can be detected and encoded into logic states D using the assumption that M > N so as to represent the bus states B in the dominant logic scheme. With reference to Table 2 shown below, the binary values are combined such that when I ≥ M > N, the first multiplexor 8 outputs a logic state of 1; when I < N < M, the first multiplexor 8 outputs a logic state of 0; and when I < M and I > N, the first multiplexor 8 outputs a logic state of Z. Iab 2 Binary Value encoded logic state D output First Second First Second from Comparator Comparator Comparator Comparator multiplexor 8 I > M I ≥ N 1 1 " 1 I < M I ≥ N 0 1 z I < M I < N 0 0 0 With reference to the comprehensive Table 3 shown below, the output of the data receiving transceiver's ternary receiver 6 consists of partially decoded logic states E which denote to a certain extent the signal state A transmitted by the driver 4 of the data transmitting transceiver 2. The partially decoded logic states E are derived through the use of a second multiplexor 9 which receives as input the logic states D output from the first multiplexor 8 in addition to the signal states A. The values of the logic states D and the signal states A are combined in the second multiplexor 9 as shown in Table 3 to obtain the logic states E: Table 3 Local Comparator Comparator Local Local signal state Signal value for value for encoded partially A driven by state A Bus state comparator comparator logic state decoded node 51 driven by B 11 12 D logic state E node 52 0 0 0 0 0 0 0 As an example, if the driver 4 of the transceiver 21 outputs a Z signal state and the driver 4 of the data transmitting transceiver 2 outputs a 0 signal state, the temary receiver 6 of the data receiving transceiver 21 will detect the presence of a 0 bus state (the resulting current being approximately -4 mA on the positive signal wire 42 and +4 mA on the negative signal wire 43, which on a doubly terminated 110 Ω characteristic impedance transmission line translates into approximately -220 mV of differential voltage, respectively). This is encoded as described above into a 0 logic state by the first multiplexor 8 and supplied to the second multiplexor 9 which also receives as input the signal state A locally driven onto the transmission channel 20 by the driver 4 of the data receiving transceiver 21. Knowing that the data receiving transceiver 21 is driving a Z signal state on the transmission channel 20 at that time, the second multiplexor 9 determines (for example, by implementing columns 1 and 6 of the above Table 3 in the multiplexor's circuitry) that the 0 logic state is caused by transceiver 2 driving a signal state of 0 on the transmission channel 20 at the same time. Hence, the output from the ternary receiver 6 is a 0 decoded logic state. In addition to being able to transmit and decode control signals in a full-duplex transmission environment, the present invention also provides a means for interrupting the half-duplex transmission of data signals between the first and second nodes 51,52 so that higher priority control signal transfers can take place. For example, if the second node 52 is in the process of a data signal transfer to the first node 51, it is desirable that the first node 51 be capable of signaling to the second node 52 that it has a higher priority control signal transfer to conduct. In such a situation, the transmitting node 52 would then terminate its data signal transfer in order to permit the first node 51 to begin its higher priority control signal transfer. In one embodiment of the present invention, this is performed by providing a preemptive signaling receiver 7 comprising either a voltage comparator or an operational amplifier in the clock transmitting transceiver of each node, i.e. in transceiver 21 of node 51 and transceiver 22 of node 52. The preemptive signaling receiver 7 of the transceiver 21 of the first node 51 is connected to the twisted pair signal lines 42,43 used for the transfer of the reference clock signal from the first node 51 to the second node 52. Similarly, the preemptive signaling receiver 7 of the transceiver 22 of the second node 52 is connected to the twisted pair signal lines 40,41 used for the transfer of the reference clock signal from the second node 52 to the first node 51. When the first node 51 must transmit priority control information to the second node 52 during a data transfer from the second node 52 to the first node 51, the first node 51 enables the driver 4 of the clock receiving transceiver 1 so that a signal state of 1 (a preemptive message) is driven on the signal lines 40,41. In this manner, the dc level of the reference clock signal on the signal lines 40,41 is shifted up from alternating bus states of 0 and 1 to alternating bus states of Z and 11 which triggers the built in voltage threshold of the comparator 14 of the preemptive signaling receiver 7 of the clock transmitting transceiver 22. The preemptive receiver 7 of the clock transmitting transceiver 22 then outputs a corresponding preemptive signal to the driving circuitry of the second node 52 to stop the transfer of data to the first node 51. In terms of currents, the current levels for a differential clock signal representing alternating signal states of 0 and 1 on the signal lines 40,41 comprise alternating values of -4 A and +4 mA on the positive signal wire 40 and alternating values of +4 mA and -4 mA on the negative signal wire 41, respectively. When the clock receiving transceiver 1 drives a signal state of 1 comprising current amplitudes of +4 mA and -4 mA on the positive wire 40 and the negative wire 41, respectively, the resulting current amplitudes on the wires 40, 41 comprise alternating values of 0 mA and +8 mA on the positive wire 40 and alternating values of 0 and -8 mA on the negative wire 41, respectively. To detect the resultant alternating bus states of Z and 11 produced by the preemptive signaling, the threshold of the comparator 14 of each preemptive signaling receiver 7 is preferably set at approximately +100 mV. Furthermore, each preemptive signaling receiver 7 also comprises a filtering circuit 15 which extracts the DC component of the differential clock signal driven on the clock lines 40,41 as is well-known in the art. The time constant of this filter 15 should be long enough to reduce the amplitude of the AC signal component below the range of the preemptive signaling receiver's threshold level, but small enough to provide an adequately short reaction time to the preemptive signal. So as to permit the voltage comparators 11,12,13,14 forming the receivers 5,6,7 of the present invention to receive the transmitted signals at the amplitude required to detect the proper bus voltage values, the present invention further provides a means for common mode shifting of the signals at the front end of the receivers 5,6,7 while providing for a voltage offset independent of the fabrication process. The voltage drop that can occur across the ground connection between the two nodes 51,52 connected via bus 45 may cause the common mode value of the received signal amplitude to fall below the common mode range of the receivers 5,6,7. Therefore, in order to obtain the proper signal amplitudes required by the comparators 11,12,13,14 and to optimize the common mode performance of the bus 45, a common mode level shift of approximately 650 mV is implemented at the front end of each receiver 5,6,7. This is performed by providing each receiver 5,6,7 of the transceivers 1,2,21,22 with the level shifting circuit 10 shown in Fig. 2 wherein the PNP bipolar junction transistors Ql and Q2 implement a common mode voltage translator in front of the voltage comparators 11,12, 13, 14. Specifically, two current sources II and 12 are disposed in parallel downstream from a common voltage bias and are respectively coupled to the emitter of each PNP transistor Ql and Q2 similarly arranged in parallel. The base of each PNP transistor Ql and Q2 is coupled to a separate signal wire, while the collectors of the PNP transistor Ql and Q2 are mutually tied to ground. The emitter of each PNP transistor Ql and Q2 is further coupled to a separate input of the voltage comparators 11,12,13,14. With this arrangement, the PNP bipolar junction transistors Ql and Q2 provide a DC level shift of approximately 650 mV over their base to emitter junctions while the current sources II and 12 provide the biasing mechanism for the PNP transistors Ql and Q2 so that the ratio between the current amplitudes of the current sources II and 12 controls the thresholds of the comparators 11,12,13,14. However, the current absolute magnitudes of the current sources II and 12 should be to a certain extent independent of process and temperature variations in order to obtain a predictable common mode level shift. For the high speed, binary receivers 5, which in this specific embodiment have a zero threshold, the two currents should be equal in amplitude (i.e., a value of approximately II = 12 = 30 μA, with the bipolar transistors Ql and Q2 being identical). In this manner, the binary receiver 5 will output a binary 1 when Va ≥ Vb and will output a binary 0 when Vb < Va. For the comparators 11,12 of the ternary receivers 6, the desired threshold is implemented by precisely controlling the ratio between the current sources II and 12 for each comparator 11,12. The current values for the first comparator 11 are II = 15 μa and 12 = 105 μa when Q2 is four times the size of Ql so that it will output a binary 1 when Va ≥ Vb + 98 mV and a binary 0 when Va < Vb + 98 mV. Similarly, die current values for the second comparator 12 are II = 105 μa and 12 = 15 μa when Ql is four times the size of Q2 such that it will output a binary 1 when Va > Vb - 98 mV and a binary 0 when Va < Vb - 98 mV. In general, the control over the ratio of two current sources II and 12 is very good in the integrated circuits manufacturing process. Specifically, in a standard CMOS manufacturing process, the front end PNP bipolar junction transistors Ql and Q2 can be implemented as vertical substrate transistors without any process modifications. The offset thus implemented will have a small variation with temperature due to the variation of the base-emitter junction voltages of the two PNP transistors with temperature. This variation can be easily canceled by implementing an opposite temperature dependency for the two bias current sources II and 12. In addition, the level shifting circuit 10 shown in Fig. 2 also permits the implementation of a single ternary receiver 6 in place of each of the binary, ternary and preemptive signaling receivers 5,6,7 for each transceiver 21,22 although this will increase the theoretical bit error rate during the arbitration transfer mode due to the high bandwidth of the ternary receiver. That is, the present invention can be modified so that both transfer modes in addition to the preemptive signaling method can be performed using a single ternary receiver 6. During the ternary arbitration transfer mode, the ternary receiver 6 operates as discussed above. Specifically, the threshold of the first comparator 11 is set to a first value corresponding to a current amplitude half way between the 1 signal state and the Z signal state, and the threshold of the second comparator 12 is set to a second value corresponding to a current amplitude half way between the 0 signal state and the Z signal state for detecting the bus states B in a simultaneous driving situation. However, in the binary data transfer mode, the ratio between the current sources II and 12 for the PNP level shifting circuit 10 placed in front of the second comparator 12 can be dynamically switched (or controlled) in order to utilize the second comparator 12 of the ternary receiver 6 as the binary receiver 5. In this manner, the threshold of the second comparator 12 is set to a third value corresponding to a current amplitude half way between the binary signal states transmitted on the bus (i.e., 0 volts) for use as the binary receiver 5. With respect to the first comparator 11, its threshold is maintained at the first value during the binary data transfer mode so as to utilize the second comparator 12 as the preemptive signaling receiver 7. However, if preemptive signaling is not required, then the first comparator 11 may simply be disabled. In this way, the overall size and complexity of the transceiver circuits can be greatly reduced through the use of a single ternary receiver 6. The increase in the theoretical bit error rate problem can be solved by reducing the bandwidth of the second voltage comparator 12 when it is used as part of the ternary receiver in the arbitration phase and then, restoring its bandwidth to its full value when it is used as a binary receiver in the data transfer phase. The bandwidth of the first voltage comparator 11 can be maintained low as it is used exclusively for receiving the control signals during the arbitration phase and the preemptive signal during the data transfer phase. According to a further embodiment of the present invention, a method and apparatus is provided for the full duplex transmission of arbitration signals in a particular multi-speed bus architecture scheme designed pursuant to the IEEE Standards Document P1394 entitled "High Performance Serial Bus" (draft 5.3vl of October 14,1992). This document is appended hereto as Appendix A for reference. The standard described in P1394 is designed to provide a high speed, serial bus having a low cost interconnection between nodes within the computer system. As shown in Fig. 3, a plurality of nodes 3 of a computer system are arbitrarily coupled together via a plurality of links 40 to form an interconnected, multi-speed bus 80. In this embodiment, each link 40 comprises a differential twisted pair, serial bus 45 as described. Additionally, each node 3 coupled to the interconnected, multi-speed serial bus comprises a physical channel interface chip (or "PHY") directly connected to the serial bus for the reception and transmission of control and data signals. Each node 3 also comprises a link layer chip (or "LLC") which transmits to and receives from the PHY both data and control signals via a fixed speed, variable size bus Y forming a scalable interface. The LLC also interfaces with the local host 25 (i.e., a microprocessor MP) of the node via a fixed speed, fixed size bus X in order to process the data received from the PHY. With such an arrangement, the interconnected pairs of bus transceivers 1,22 and 2,21 (shown in Fig. 1) of the present invention are respectively utilized in adjacent nodes 3 of the system. In this manner, the transmission of data signals between PHY's of adjacent nodes 3 is performed using the binary transfer mode and the transmission of control signals is performed using the ternary transfer mode as described above. Furthermore, in the specific PI 394 implementation, the PHY circuits are supplied with power via the links 40 which contain, in addition to the communication channels 20, a voltage distribution line and a ground connection line. Although this power distribution architecture may cause a significant voltage difference in the ground potential between two adjacent nodes 3, the use of the level shifting circuits 10 of the present invention advantageously solves this problem. While the present invention has been described in conjunction with the above-embodiments, it is evident that numerous alternatives, depictions, variations and uses will be apparent to those skilled in the art in light of the foregoing description. As previously noted, it would be obvious to one skilled in the art to utilize the bus transceivers including the dominant logic scheme of the present invention in any type of bus which performs baseband data signal transfers despite the number of transmission channels in the bus, the type of communication channel (i.e., single ended or differential) or the specific number of signaling levels used.

Claims: CLAIMS: 1. A method for enabling the full duplex transmission of messages on a bus connecting a first transceiver and a second transceiver, the bus comprising at least one transmission channel for the simultaneous transmission of messages comprising signal states corresponding to current amplitudes driven on the bus by the first transceiver and the second transceiver, the method comprising: simultaneously transmitting on the bus signal states by the first transceiver and signal states by the second transceiver for reception by at least one ternary receiver of the second transceiver and at least one ternary receiver of the first transceiver, respectively, the ternary receiver of each transceiver comprising at least two binary receivers for receiving resultant current amplitudes created on the bus; outputting binary values from the binary receivers of the ternary receiver of each transceiver representing the approximate magnitude of the resultant current amplitude on the bus; supplying the binary values output from the binary receivers of the ternary receiver of each transceiver and the signal states being driven locally by the respective transceivers as input to logic means of the ternary receiver of each transceiver; and deriving as output from the logic means of the ternary receiver of each the first and second transceivers reconstructed messages representing the messages driven on the bus by the second transceiver and the messages driven on the bus by the first transceiver, respectively. 2. The method according to claim 1, wherein the method further comprises before the simultaneous transmission of signal states on the bus the step of choosing the current amplitudes of at least a first signal state, a second signal state and a third signal state such that the first and second signal states have current amplitudes of opposite polarity and a combination of the current amplitudes of the first and second signal states produces the current amplitude of the third signal state. 3. The method according to claim 2, wherein the step of choosing the current amplitudes of at least a first signal state, a second signal state and a third signal state comprises the step of selecting the current amplitudes such that the first and second signal states have equal current amplitudes opposite in sign and the third signal state has a zero current amplitude for use as a bus idle state. 4. The method according to claim 1, wherein the steps of supplying the binary values output from the binary receivers of the ternary receiver of each transceiver and the signal states being driven locally by the respective transceivers as input to logic means of the ternary receiver of each transceiver and deriving as output from the logic means of the ternary receiver of each the first and second transceivers reconstructed messages representing the messages driven on the bus by the second transceiver and the messages driven on the bus by the first transceiver, respectively, comprises the steps of: supplying the binary values output from the binary receivers of the ternary receiver of each transceiver as input to a first multiplexor of the ternary receiver of each transceiver; deriving as output from the first multiplexor of the ternary receiver of each transceiver logic states representing the resultant current amplitudes created on the bus; supplying as input to a second multiplexor of the ternary receiver of each the first and second transceivers the logic states output from the first multiplexor of each the first and second transceivers, respectively, in addition to the signal states being transmitted by the first transceiver and the signal states being transmitted by the second transceiver, respectively; and deriving as output from the second multiplexor of the ternary receiver of each the first and second transceivers partially decoded logic states representing the messages being driven on the bus by the second transceiver and the messages being driven on the bus by the first transceiver, respectively. 5. The method according to claim 4, wherein the step of outputting binary values from the binary receivers of the ternary receiver of each transceiver representing the approximate magnitude of the resultant current amplitude on the bus comprises the steps of: deriving as output from a first binary receiver comprising a first voltage comparator having a first voltage threshold a binary value comprising a first binary value when the current amplitude is greater than or equal to the first voltage threshold, and a second binary value when the current amplitude is less than the first voltage threshold; and deriving as output from a second binary receiver comprising a second voltage comparator having a second voltage threshold a binary value comprising the first binary value when the current amplitude is greater than or equal to the second voltage threshold, and the second binary value when the current amplitude is less than the second voltage threshold. 6. The method according to claim 5, wherein the step of deriving as output from the first multiplexor of the ternary receiver of each transceiver the logic states representing the resultant current amplitudes created on the bus comprises the steps of: outputting a first logic state when two first binary values are input to the first multiplexor; outputting a second logic state when two second binary values are input to the first multiplexor; and outputting a third logic state when the first binary value and the second binary value are input to the first multiplexor. 7. The method according to claim 6, wherein the step of deriving as output from the second multiplexor of the ternary receiver of each the first and second transceivers partially decoded logic states representing the messages being driven on the bus by the second transceiver and the messages being driven on the bus by the first transceiver, respectively, comprises the steps of: outputting the second signal state in response to an input of the second logic state and the second signal state; outputting the second signal state in response to an input of the third logic state and the first signal state; outputting the second signal state in response to an input of the second logic state and the third signal state; outputting the first signal state in response to an input of the third logic state and the second signal state; outputting the first signal state in response to an input of the first logic state and the first signal state; outputting the first signal state in response to an input of the first logic state and the third signal state; outputting the second signal state in response to an input of the second logic state and the third signal state; outputting the first signal state in response to an input of the first logic state and the third signal state; and outputting the third signal state in response to an input of the third logic state and the third signal state. 8. A method for enabling the full duplex transmission of messages on a bus connecting a first transceiver and a second transceiver, the bus comprising at least one transmission channel for the simultaneous transmission of messages comprising signal states corresponding to currents driven on the bus by the first transceiver and the second transceiver, the method comprising: choosing the currents of a first signal state, a second signal state, a third signal state, a fourth signal state and a fifth signal state such that the first and fifth signal states have equal current amplitudes opposite in sign, the second and fourth signal states have equal current amplitudes opposite in sign and the third signal state has a zero current amplitude, the current amplitudes of the first and fifth signal states being greater than the current amplitudes of the second and fourth signal states; simultaneously transmitting on the bus signal states by the first transceiver and signal states by the second transceiver for reception by at least one ternary receiver of the second transceiver and at least one ternary receiver of the first transceiver, respectively, the ternary receiver of each transceiver comprising at least four binary receivers for receiving resultant current amplitudes created on the bus and outputting binary values representing the approximate magnitude of the resultant current amplitude on the bus; supplying the binary values output from the binary receivers of the ternary receiver of each transceiver and the signal states being driven locally by the respective transceivers as input to logic means of the temary receiver of each transceiver; and deriving as output from the logic means of the ternary receiver of each the first and second transceivers reconstructed messages representing the messages driven on the bus by the second transceiver and the messages driven on the bus by the first transceiver, respectively. 9. The method according to claim 8, wherein the steps of supplying the binary values output from the binary receivers of the ternary receiver of each transceiver and the signal states being driven locally by the respective transceivers as input to logic means of the ternary receiver of each transceiver and deriving as output from the logic means of the ternary receiver of each the first and second transceivers reconstructed messages representing the messages driven on the bus by the second transceiver and the messages driven on the bus by the first transceiver, respectively, comprises the steps of: supplying the binary values output from the binary receivers of the ternary receiver of each transceiver as input to a first multiplexor of the ternary receiver of each transceiver; deriving as output from the first multiplexor of the ternary receiver of each transceiver logic states representing the resultant current amplitudes created on the bus; supplying as input to a second multiplexor of the ternary receiver of each the first and second transceivers the logic states output from the first multiplexor of each the first and second transceivers, respectively, in addition to the signal states being transmitted by the first transceiver and the signal states being transmitted by the second transceiver, respectively; and deriving as output from the second multiplexor of the ternary receiver of each the first and second transceivers partially decoded logic states representing the messages being driven on the bus by the second transceiver and the messages being driven on the bus by the first transceiver, respectively. 10. In a computer system having a plurality of devices arbitrarily coupled together via a plurality of twisted pair, differential busses forming an interconnected bus for the transfer of differential signals between the devices of the computer system, each device of the computer system comprising a node coupled to at least one of the plurality of differential busses, a method is provided for enabling the full duplex transmission of messages on one of the differential busses connecting a first transceiver and a second transceiver, the bus comprising two pairs of signal wires each pair having a positive and a negative signal wire forming a transmission channel for the simultaneous transmission of messages comprising signal states corresponding to current amplitudes driven on each pair of signal wires by the first transceiver and the second transceiver, the method comprising: choosing the current amplitudes of a first signal state, a second signal state and a third signal state such that the first signal state comprises a first positive current amplitude driven on the positive signal wire and a first negative current amplitude driven on the negative signal wire, the second signal state comprises the first positive current amplitude driven on the negative signal wire and the first negative current amplitude driven on the positive signal wire and the third signal state comprises a zero current amplitude driven on both the positive and negative signal wires; simultaneously transmitting on the bus signal states by the first transceiver and signal states by the second transceiver for reception by at least one ternary receiver of the second transceiver and at least one temary receiver of the first transceiver, respectively, the temary receiver of each transceiver comprising at least two binary receivers for receiving resultant current amplitudes created on each the positive and negative signal wires; outputting binary values from the binary receivers of the ternary receiver of each transceiver representing the approximate magnitude of the resultant current amplitude on the bus; supplying the binary values output from the binary receivers of the ternary receiver of each transceiver and the signal states being driven locally by the respective transceivers as input to logic means of the ternary receiver of each transceiver; and deriving as output from the logic means of the ternary receiver of each the first and second transceivers reconstructed messages representing the messages driven on the bus by the second transceiver and the messages driven on the bus by the first transceiver, respectively. 11. The method according to claim 10, wherein the steps of supplying the binary values output from the binary receivers of the ternary receiver of each transceiver and the signal states being driven locally by the respective transceivers as input to logic means of the ternary receiver of each transceiver and deriving as output from the logic means of the ternary receiver of each the first and second transceivers reconstructed messages representing the messages driven on the bus by the second transceiver and the messages driven on the bus by the first transceiver, respectively, comprises the steps of: supplying the binary values output from the binary receivers of the ternary receiver of each transceiver as input to a first multiplexor of the ternary receiver of each transceiver; deriving as output from the first multiplexor of the ternary receiver of each transceiver logic states representing the resultant current amplitudes created on each signal wire; supplying as input to a second multiplexor of the ternary receiver of each the first and second transceivers the logic states output from the first multiplexor of each the first and second transceivers, respectively, in addition to the signal states being transmitted by the first transceiver and the signal states being transmitted by the second transceiver, respectively; and deriving as output from the second multiplexor of the ternary receiver of each the first and second transceivers partially decoded logic states representing the messages being driven on the bus by the second transceiver and the messages being driven on the bus by the first transceiver, respectively. 12. The method according to claim 11, wherein the step of outputting binary values from the binary receivers of the ternary receiver of each transceiver representing the approximate magnitude of the resultant current amplitude on each of the positive and negative signal wires comprises the steps of: deriving as output from a first binary receiver a first binary value when the resultant current amplitude on the positive wire is approximately equal to the first positive current amplitude and the resultant current amplitude on the negative wire is approximately equal to the first negative current amplitude, a second binary value when the resultant current amplitude on the positive wire is approximately equal to the first negative current amplitude and the resultant current amplitude on the negative wire is approximately equal to the first positive current amplitude, and the second binary value when the resultant current amplitude on both the positive and negative signal wires is approximately zero; and deriving as output from a second binary receiver the first binary value when the resultant current amplitude on the positive wire is approximately equal to the first positive current amplitude and the resultant current amplitude on the negative wire is approximately equal to the first negative current amplitude, the second binary value when the resultant current amplitude on the positive wire is approximately equal to the first negative current amplitude and the resultant current amplitude on the negative wire is approximately equal to the first positive current amplitude, and the first binary value when the resultant current amplitude on both the positive and negative signal wires is approximately zero. 13. The method according to claim 12, wherein the step of deriving as output from the first multiplexor of the ternary receiver of each transceiver the logic states representing the resultant current amplitudes created on the bus comprises the steps of: outputting a first logic state when two first binary values are input to the first multiplexor; outputting a second logic state when two second binary values are input to the first multiplexor; and outputting a third logic state when the first binary value and the second binary value are input to the first multiplexor. 14. The method according to claim 13, wherein the step of deriving as output from the second multiplexor of the ternary receiver of each the first and second transceivers partially decoded logic states representing the messages being driven on the bus by the second transceiver and the messages being driven on the bus by the first transceiver, respectively, comprises the steps of: outputting the second signal state in response to an input of the second logic state and the second signal state; outputting the second signal state in response to an input of the third logic state and the first signal state; outputting the second signal state in response to an input of the second logic state and the third signal state; outputting the first signal state in response to an input of the third logic state and the second signal state; outputting the first signal state in response to an input of the first logic state and the first signal state; outputting the first signal state in response to an input of the first logic state and the third signal state; outputting the second signal state in response to an input of the second logic state and the third signal state; outputting the first signal state in response to an input of the first logic state and the third signal state; and outputting the third signal state in response to an input of the third logic state and the third signal state. 15. In a bus architecture having at least a first communications node and a second communications node coupled together via a bus having at least one transmission channel, each node having at least a first transceiver for enabling full duplex transmission of control signals and half duplex transmission of data signals between the nodes, the first transceiver of the first node comprising: a driver coupled to the bus for driving on the bus current amplitudes corresponding to signal states, the signal states comprising at least a first signal state, a second signal state and a third signal state; a first binary receiver coupled to the bus for receiving data signals driven on the bus by the first transceiver of the second node during a data transfer phase, the data signals comprising a plurality of the first and second signal states; and a ternary receiver coupled to the bus for receiving control signals driven on the bus by the first transceiver of the second node, the control signals comprising a plurality of the first, second and third signal states, the ternary receiver having two binary receivers for detecting resultant current amplitudes created on the bus during simultaneous driving of control signals by the nodes during an arbitration transfer phase and logic means for combining the resultant current amplitudes on the bus with the signal states driven by the first transceiver of the first node to output reconstructed control signals representing the control signals driven on the bus by the first transceiver of the second node. 16. The transceiver of claim 15, wherein the first and second signal states have current amplitudes of opposite polarity and a combination of the current amplitudes of the first and second signal states produces the current amplitude of the third signal state. 17. The transceiver claim 16, wherein the first and second signal states have equal current amplitudes opposite in sign and the third signal state has a zero current amplitude for use as a bus idle state. 18. The transceiver of claim 15, wherein the logic means of the temary receiver comprises at least a first multiplexor and a second multiplexor, the first multiplexor receiving as input binary values output by the binary receivers of the temary receiver and outputting logic states representing the approximate magnitude of the resultant current amplitudes on the bus, the second multiplexor receiving the logic states output by the first multiplexor in addition to the signal states driven on the bus by the first transceiver of the first node and outputting partially decoded logic states representing the control signals driven by the first transceiver of the second node. 19. The transceiver of claim 18, wherein the binary receivers of the ternary receiver comprise a first voltage comparator having a first voltage threshold and a second voltage comparator having a second voltage threshold, respectively, the first voltage comparator outputting a first binary value when the current amplitude is greater than or equal to the first voltage threshold and a second binary value when the current amplitude is less than the first voltage threshold, the second voltage comparator outputting the first binary value when the current amplitude is greater than or equal to the second voltage threshold and the second binary value when the current amplitude is less than the second voltage threshold. 20. The transceiver of claim 19, wherein the first multiplexor of the temary receiver outputs a first logic state when two first binary values are input to the first multiplexor, a second logic state when two second binary values are input to the first multiplexor and a third logic state when the first binary value and the second binary value are input to the first multiplexor. 21. The transceiver of claim 20, wherein the second multiplexor of the temary receiver outputs the second signal state in response to an input of the second logic state and the second signal state, outputs the second signal state in response to an input of the third logic state and the first signal state, outputs the second signal state in response to an input of the second logic state and the third signal state, outputs the first signal state in response to an input of the third logic state and the second signal state, outputs the first signal state in response to an input of the first logic state and the first signal state, outputs the first signal state in response to an input of the first logic state and the third signal state, outputs the second signal state in response to an input of the second logic state and the third signal state, outputs the first signal state in response to an input of the first logic state and the third signal state and outputs the third signal state in response to an input of the third logic state and the third signal state. 22. The bus architecture of claim 15, wherein each of the first and second nodes further comprise a second transceiver identical to the first transceiver, the first transceiver of the first node coupled to the first transceiver of the second node via a first transmission channel of the bus, the second transceiver of the first node coupled to the second transceiver of the second node via a second transmission channel of the bus, the first and second transmission channels conveying data signals and reference clock signals, respectively, during a first data transfer phase where data is transferred from the first node to the second node, the first and second transmission channels conveying reference clock signals and data signals, respectively, during a second data transfer phase where data is transferred from the second node to the first node, both the first and second transmission channels further conveying control signals in a full duplex mode during a control transfer phase where control signals are simultaneously transferred between the nodes. 23. The bus architecture of claim 22, wherein the second transceiver of the first node and the first transceiver of the second node further comprise a preemptive signaling receiver coupled to the second and first transmission channels of the bus, respectively, for detecting a preemptive message driven on the transmission channels by the second transceiver of the second node and the first transceiver of the first node when the first and second data transfer phases are to be terminated for the transmission of priority control signals, the preemptive signaling receiver of the second transceiver of the first node and the first transceiver of the second node outputting to the driver of the first transceiver of the first node and the driver of the second transceiver of the second node a disable signal for disabling the respective drivers upon detection of the preemptive message. 24. The bus architecture of claim 23, wherein the preemptive message comprises a signal state having a positive current amplitude for increasing a current amplitude of the reference clock signal when driven on the first and second transmission channels during the second and first data transfer phases, respectively, the preemptive signaling receivers each comprising a voltage comparator having a voltage threshold for outputting the disable signal upon detection of an increase in the current amplitude of the reference clock signal. 25. In a bus architecture having at least a first communications node and a second communications node coupled together via a bus, the bus having a first transmission channel and a second transmission channel for the transmission of data signals and reference clock signals, respectively, during a first data transfer phase where data is transferred from the first node to the second node and for the transmission of reference clock signals and data signals. respectively, during a second data transfer phase where data is transferred from the second node to the first node, both the first and second transmission channels further utilized for the transmission of control signals during a control transfer phase where control signals are simultaneously transmitted between the nodes, the bus architecture comprising: a first transceiver of the first node coupled to a corresponding second transceiver of the second node via the first transmission channel; a third transceiver of the second node coupled to a corresponding fourth transceiver of the first node via the second transmission channel; and each of the first, second, third and fourth transceivers comprising a driver coupled to a respective transmission channel of the bus for transmitting the data and reference clock signals during respective data transfer phases and the control signals during control transfer phases, the data signals, clock reference signals and control signals comprising signal states corresponding to current amplitudes driven on the transmission channel, a binary receiver coupled to the respective transmission channel for receiving data signals and reference clock signals transmitted in a half duplex mode during respective data transfer phases. a ternary receiver coupled to the respective transmission channel for receiving control signals transmitted in a full duplex mode during control transfer phases, the ternary receiver having two binary receivers for detecting resultant current amplitudes created on the respective transmission channel during simultaneous driving of control signals via the respective transmission channel and logic means for combining the resultant current amplitudes formed on the respective transmission channel of the bus with the signal states locally driven by the respective transceiver to output reconstructed control signals representing the control signals driven on the respective transmission channel by the corresponding transceiver. 26. The bus architecture of claim 25, wherein the second and fourth transceivers further comprise a preemptive signaling receiver coupled to the first and second transmission channels of the bus, respectively, for detecting a preemptive message driven on the transmission channels by the driver of the first transceiver and the driver of the third transceiver, respectively, when the second and first data transfer phases are to be respectively terminated for the transmission of priority control signals, the preemptive signaling receiver of the second and fourth transceivers outputting to the driver of the third transceiver and the driver of the first transceiver a disable signal for disabling the respective drivers upon detection of the preemptive message. 27. The bus architecture of claim 26, wherein the preemptive message comprises a signal state having a positive current amplitude for increasing a current amplitude of the reference clock signal when driven on the first and second transmission channels during the second and first data transfer phases, respectively, the preemptive signaling receivers each comprising a voltage comparator having a voltage threshold for outputting the disable signal upon detection of an increase in the current amplitude of the reference clock signal.

Inventors: OPRESCU FLORIN, VAN BRUNT ROGER, OPRESCU, FLORIN, VAN BRUNT, ROGER
Applicants: APPLE COMPUTER, APPLE COMPUTER, INC.
Application year: 1994
Main classification: H04L5
IPC codes: G06F3, H04L5, H04L12, H04L25